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2013
Hopkinson, BM, Seegers B, Hatta M, Measures CI, Mitchell BG, Barbeau KA.  2013.  Planktonic C:Fe ratios and carrying capacity in the southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:102-111.   10.1016/j.dsr2.2012.09.001   AbstractWebsite

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July-August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4 x 10(5) (mol:mol) for the radiotracer method and 1.7 x 10(5) for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. (c) 2012 Elsevier Ltd. All rights reserved.

Jiang, MS, Barbeau KA, Selph KE, Measures CI, Buck KN, Azam F, Mitchell BG, Zhou M.  2013.  The role of organic ligands in iron cycling and primary productivity in the Antarctic Peninsula: A modeling study. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:112-133.   10.1016/j.dsr2.2013.01.029   AbstractWebsite

Iron (Fe) is the limiting nutrient for primary productivity in the Southern Ocean, with much of the dissolved iron (dFe) bound to organic ligands or colloids. A Fe model for the Southern Ocean (SOFe) is developed to understand the role of bacteria and organic ligands in controlling Fe cycling and productivity. The model resolves the classical food web and microbial loop, including three types of nutrients (N, Si, Fe) and two types of Fe ligands. Simulations of the zero-dimensional (0-D) model are calibrated with detailed results of shipboard grow-out incubation experiments conducted with Antarctic Peninsula phytoplankton communities during winter 2006 to provide the best estimate of key biological parameters. Then a one-dimensional (1-D) model is developed by coupling the biological model with the Regional Oceanic Modeling System (ROMS) for a site on the Antarctic Peninsula shelf, and the model parameters are further calibrated with data collected from two surveys (summer 2004 and winter 2006) in the area. The results of the numerical simulations agree reasonably well with observations. An analysis of the 1-D model results suggests that bacteria and organic ligands may play an important role in Fe cycling, which can be categorized into a relatively fast mode within the euphotic zone dominated by photo-reactions (summer d Fe residence time about 600 days) and complexation and a slow mode below with most of the dFe biologically complexed (summer dFe residence time > 10 years). The dFe removal from the euphotic zone is dominated by colloidal formation and further aggregations with additional contribution from biological uptake, and an increase of organic ligands would reduce Fe export. The decrease of Fe removal rate over depth is due to the continuous dissolution and remineralization of particulate Fe. A number of sensitivity experiments are carried out for both O-D and 1-D models to understand the importance of photo-reactive processes in primary productivity, bacterial activity, Fe speciation, and dFe residence time within the euphotic zone. The bio-availability of ligand-bound Fe (FeL) is critical to modeled high primary productivity, which is consistent with both shipboard measurements and field observations. In addition, model productivity is sensitive to photoreaction rates if FeL is not directly available for phytoplankton uptake. (C) 2013 Elsevier Ltd. All rights reserved.

Bundy, RM, Barbeau KA, Buck KN.  2013.  Sources of strong copper-binding ligands in Antarctic Peninsula surface waters. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:134-146.   10.1016/j.dsr2.2012.07.023   AbstractWebsite

Copper-binding organic ligands were measured during austral winter in surface waters around the Antarctic Peninsula using competitive ligand exchange-adsorptive cathodic stripping voltammetry with multiple analytical windows. Samples were collected from four distinct water masses including the Antarctic Circumpolar Current, Southern Antarctic Circumpolar Current Front, Bransfield Strait, and the shelf region of the Antarctic Peninsula. Strong copper-binding organic ligands were detected in each water mass. The strongest copper-binding ligands were detected at the highest competition strength in the Antarctic Circumpolar Current, with an average conditional stability constant of logK(CuL,Cu2+)(cond) = 16.00 +/- 0.82. The weakest ligands were found at the lowest competition strength in the shelf region with logK(CuL,Cu2+)(cond) = 12.68 +/- 0.48. No ligands with stability constants less than logK(CuL,Cu2+)(cond) = 13.5 were detected in the Antarctic Circumpolar Current at any competition strength, suggesting a shelf source of weaker copper-binding ligands. Free, hydrated copper ion concentrations, the biologically available form of dissolved copper, were less than 10(-14) M in all samples, approaching levels that may be limiting for some types of inducible iron acquisition. (C) 2012 Elsevier Ltd. All rights reserved.

2012
Landry, MR, Ohman MD, Goericke R, Stukel MR, Barbeau KA, Bundy R, Kahru M.  2012.  Pelagic community responses to a deep-water front in the California Current Ecosystem: overview of the A-Front Study. Journal of Plankton Research. 34:739-748.   10.1093/plankt/fbs025   AbstractWebsite

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front.

Roe, KL, Barbeau K, Mann EL, Haygood MG.  2012.  Acquisition of iron by Trichodesmium and associated bacteria in culture. Environmental Microbiology. 14:1681-1695.   10.1111/j.1462-2920.2011.02653.x   AbstractWebsite

Trichodesmium colonies contain an abundant microbial consortium that is likely to play a role in nutrient cycling within the colony. This study used laboratory cultures of Trichodesmium and two genome-sequenced strains of bacteria typical of Trichodesmium-associated microbes to develop an understanding of the cycling of iron, a potentially limiting micronutrient, within Trichodesmium colonies. We found that the ferric siderophores desferrioxamine B and aerobactin were not readily bioavailable to Trichodesmium, relative to ferric chloride or citrate-associated iron. In contrast, the representative bacterial strains we studied were able to acquire iron from all of the iron sources, implying that naturally occurring Trichodesmium-associated bacteria may be capable of utilizing a more diverse array of iron sources than Trichodesmium. From the organism-specific uptake data collected in this study, a theoretical Trichodesmium colony was designed to model whole colony iron uptake. The bacteria accounted for most (> 70%) of the iron acquired by the colony, highlighting the importance of determining organism-specific uptake in a complex environment. Our findings suggest that, although they may share the same micro-environment, Trichodesmium and its colony-associated microbial cohort may differ substantially in terms of iron acquisition strategy.

Buck, KN, Moffett J, Barbeau KA, Bundy RM, Kondo Y, Wu JF.  2012.  The organic complexation of iron and copper: an intercomparison of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques. Limnology and Oceanography-Methods. 10:496-515.   10.4319/lom.2012.10.496   AbstractWebsite

Characterization of the speciation of iron and copper is an important objective of the GEOTRACES Science Plan. To incorporate speciation measurements into such a multinational program, standard practices must be adopted that allow data from multiple labs to be synthesized. Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) is the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater. The determination of concentrations and conditional stability constants of metal-binding ligands is particularly challenging, as results can be influenced both by experimental conditions and interpretation of titration data. Here, we report an investigation between four laboratories to study the speciation of iron and copper using CLE-ACSV. Samples were collected on the GEOTRACES II intercomparison cruise in the North Pacific Ocean in May 2009 at 30 degrees N, 140 degrees W. This intercomparison was carried out shipboard and included an assessment of the viability of sample preservation by freezing. Results showed that consensus values could be obtained between different labs, but that some existing practices were problematic and require further attention in future work. A series of recommendations emerged from this study that will be useful in implementing multi-investigator programs like GEOTRACES.

Hopkinson, BM, Barbeau KA.  2012.  Iron transporters in marine prokaryotic genomes and metagenomes. Environmental Microbiology. 14:114-128.   10.1111/j.1462-2920.2011.02539.x   AbstractWebsite

In the pelagic environment, iron is a scarce but essential micronutrient. The iron acquisition capabilities of selected marine bacteria have been investigated, but the recent proliferation of marine prokaryotic genomes and metagenomes offers a more comprehensive picture of microbial iron uptake pathways in the ocean. Searching these data sets, we were able to identify uptake mechanisms for Fe3+, Fe2+ and iron chelates (e.g. siderophore and haem iron complexes). Transport of iron chelates is accomplished by TonB-dependent transporters (TBDTs). After clustering the TBDTs from marine prokaryotic genomes, we identified TBDT clusters for the transport of hydroxamate and catecholate siderophore iron complexes and haem using gene neighbourhood analysis and co-clustering of TBDTs of known function. The genomes also contained two classes of siderophore biosynthesis genes: NRPS (non-ribosomal peptide synthase) genes and NIS (NRPS Independent Siderophore) genes. The most common iron transporters, in both the genomes and metagenomes, were Fe3+ ABC transporters. Iron uptake-related TBDTs and siderophore biosynthesis genes were less common in pelagic marine metagenomes relative to the genomic data set, in part because Pelagibacter ubique and Prochlorococcus species, which almost entirely lacked these Fe uptake systems, dominate the metagenomes. Our results are largely consistent with current knowledge of iron speciation in the ocean, but suggest that in certain niches the ability to acquire siderophores and/or haem iron chelates is beneficial.

King, AL, Buck KN, Barbeau KA.  2012.  Quasi-Lagrangian drifter studies of iron speciation and cycling off Point Conception, California. Marine Chemistry. 128:1-12.   10.1016/j.marchem.2011.11.001   AbstractWebsite

The distribution and speciation of dissolved Fe (dFe) were measured during four quasi-Lagrangian drogued drifter studies (similar to 4 d duration each) that were conducted in the southern California Current System in May 2006 and April 2007. Three of the four drifter studies were within the coastal upwelling regime and one drifter study was in a warm-core anticyclonic eddy. Incubation bottle experiments were also conducted to determine the degree of phytoplankton Fe limitation and to assess changes in the concentration of Fe-binding ligands. In the coastal upwelling drifter studies, in situ dFe (1.4-1.8 nM) and macronutrients were initially high and declined over time. Fe addition incubation experiments indicated that the phytoplankton community was not Fe limited at the beginning of the coastal upwelling drifter experiments (when mu M nitrate:nM dFe ratios were similar to 7-8). By the end of two of the three drifter studies (when mu M nitrate:nM dFe ratios were similar to 12-19), Fe addition resulted in larger nitrate and silicic acid drawdown, and larger accumulations in chlorophyll a, particulate organic carbon and nitrogen, and diatom and dinoflagellate-specific carotenoid pigments. Fe speciation was measured in situ in three of the four drifter studies with stronger L-1-type ligands found to be present in excess of dFe in all samples. In Fe speciation incubation experiments. L-1-type ligand production was observed in conjunction with phytoplankton growth under Fe-limiting conditions. The results presented here support and add a quasi-Lagrangian perspective to previous observations of dFe and macronutrient cycling over space and time within the California coastal upwelling regime, including Fe limitation within the phytoplankton community in this region and the biological production of Fe-binding ligands concomitant with Fe limitation. (C) 2011 Elsevier B.V. All rights reserved.

2011
King, AL, Barbeau KA.  2011.  Dissolved iron and macronutrient distributions in the southern California Current System. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006324   AbstractWebsite

The distribution of dissolved iron in the southern California Current System (sCCS) is presented from seven research cruises between 2002 and 2006. Dissolved iron concentrations were generally low in most of the study area (<0.5 nM), although high mixed layer and water column dissolved iron concentrations (up to 8 nM) were found to be associated with coastal upwelling, both along the continental margin and some island platforms. A significant supply of iron was probably not from a deep remineralized source but rather from the continental shelf and bottom boundary layer as identified in previous studies along the central and northern California coast. With distance offshore, dissolved iron decreased more rapidly relative to nitrate in a transition zone 10-250 km offshore during spring and summer, resulting in relatively high ratios of nitrate: dissolved iron. Higher nitrate: dissolved iron ratios could be the result of utilization and scavenging in addition to an overall lower supply of iron relative to nitrate in the offshore transition zones. The low supply of iron leads to phytoplankton iron limitation and a depletion in silicic acid relative to nitrate in the coastal upwelling and transition zones of the sCCS.

2010
Buck, KN, Selph KE, Barbeau KA.  2010.  Iron-binding ligand production and copper speciation in an incubation experiment of Antarctic Peninsula shelf waters from the Bransfield Strait, Southern Ocean. Marine Chemistry. 122:148-159.   10.1016/j.marchem.2010.06.002   AbstractWebsite

The evolution of dissolved iron (Fe) and copper (Cu) speciation was followed through a simulated spring bloom event in a 15-day incubation experiment of natural seawater collected during austral winter from high macronutrient high Fe waters of Bransfield Strait in the Southern Ocean. The incubation experiment included unamended bottles as well as Fe additions using the stable isotope of Fe, Fe-57. as inorganic ((FeCl3)-Fe-57) and organic (Fe-57-aerobactin, Fe-57-desferrioxamine B) amendments. Exposure to summer light conditions resulted in substantial growth for all treatments, mimicking the initiation of a spring bloom. The addition of Fe resulted in a 30% increase in phytoplankton biomass over unamended controls by day 15, indicating that the unamended waters became Fe limited despite initially elevated dissolved Fe concentrations. Dissolved Cu and Cu speciation remained largely unchanged for all treatments of the incubation, with Cu speciation dominated by exceedingly strong Cu-binding ligands (log K-CuL1.Cu2+(Cond) similar to 16) and low resultant Cu2+ concentrations (10(-16.3 +/- 0.3) mol L-1). In only the unamended light bottles, strong Fe-binding ligands were produced over the course of the experiment. The observed production of strong Fe-binding ligands in the control bottles that became Fe-limited supports the important role of biologically produced siderophore-type natural ligands in the marine Fe cycle. (C) 2010 Elsevier B.V. All rights reserved.

Dupont, CL, Buck KN, Palenik B, Barbeau K.  2010.  Nickel utilization in phytoplankton assemblages from contrasting oceanic regimes. Deep-Sea Research Part I-Oceanographic Research Papers. 57:553-566.   10.1016/j.dsr.2009.12.014   AbstractWebsite

In most oceanic environments, dissolved nickel (Ni) concentrations are drawn clown in surface waters with increasing concentrations at depth, implying a role for biology in the geochemical distribution of Ni Studies with phytoplankton isolates from the surface ocean have established the biochemical roles of Ni in the assimilation of urea and oxidative defense To determine if these requirements are relevant in natural marine planktonic assemblages, bottle-based fertilization experiments were used to test the effects of low-level additions of Ni. urea, or both Ni and urea to surface waters at several locations offshore of Peru and California, as well as in the Gulf of California Urea and Ni(+) urea additions consistently promoted phytoplankton growth relative to control and +Ni treatments, except in a coastal upwelling site and Peruvian water. No effect was observed in the upwelling site, but in Peruvian waters urea additions resulted in increased phytoplankton pigments and phosphate drawdown only when Ni was added concurrently, suggesting a biochemically dependent Ni-urea colimitation In the Gulf of California, Ni additions without urea resulted in increased abundances of cyanobacteria, picoeukaryotes, and the corresponding pigments As urea additions showed the overall phytoplankton community was also urea-limited, it appears that the cyanobactena and potentially the picoeukaryotes were colimited by Ni and urea in a biochemically independent fashion. In parallel, radiotracer-based uptake experiments were used to study the kinetics and spatial variation of biological Ni assimilation. In these experiments, the added radiotracer rarely equilibrated with the natural Ni present, precluding estimates a determination of in situ Ni uptake rates and suggesting that much of the natural Ni was not bioavailable. The lack of equilibration likely did not preclude the measurement of community Ni uptake kinetics, nor the comparison of measured rates between locations The highest V(max)K(p)(-1) values, which reflect a competitive advantage in Ni acquisition at low concentrations, were observed in stratified nitrogen-deplete communities, potentially linking Ni and nitrogen biogeochemistry in a manner consistent with the biochemical utilization of Ni. Overall, uptake rates were higher in the euphotic rather than non-euphotic zone communities, directly reconciling the nutrient-like depth profile of Ni The Ni uptake rates observed at the nitrate-replete Fe-deplete Peru stations were an order of magnitude lower than the other sites This result agrees with calculations suggesting that saturation of the cell surface with Ni and iron (Fe) transporters may limit uptake rates in low Fe waters. (C) 2010 Elsevier Ltd. All rights reserved

2008
Hopkinson, BM, Roe KL, Barbeau KA.  2008.  Heme uptake by Microscilla marina and evidence for heme uptake systems in the genomes of diverse marine bacteria. Applied and Environmental Microbiology. 74:6263-6270.   10.1128/aem.00964-08   AbstractWebsite

The ability to acquire diverse and abundant forms of iron would be expected to confer a survival advantage in the marine environment, where iron is scarce. Marine bacteria are known to use siderophores and inorganic iron, but their ability to use heme, an abundant intracellular iron form, has only been examined preliminarily. Microscilla marina, a cultured relative of a bacterial group frequently found on marine particulates, was used as a model organism to examine heme uptake. Searches of the genome revealed analogs to known heme transport proteins, and reverse transcription-quantitative PCR analysis of these genes showed that they were expressed and upregulated under iron stress and during growth on heme. M. marina was found to take up heme-bound iron and could grow on heme as a sole iron source, supporting the genetic evidence for heme transport. Similar putative heme transport components were identified in the genomes of diverse marine bacteria. These systems were found in the genomes of many bacteria thought to be particle associated but were lacking in known free-living organisms (e.g., Pelagibacter ubique and marine cyanobacteria). This distribution of transporters is consistent with the hydrophobic, light-sensitive nature of heme, suggesting that it is primarily available on phytoplankton or detritus or in nutrient-rich environments.

Hopkinson, BM, Barbeau KA.  2008.  Interactive influences of iron and light limitation on phytoplankton at subsurface chlorophyll maxima in the eastern North Pacific. Limnology and Oceanography. 53:1303-1318.   10.4319/lo.2008.53.4.1303   AbstractWebsite

The roles of iron and light as limiting and colimiting factors for phytoplankton growth in subsurface chlorophyll maxima (SCMs) were investigated in mesotrophic to oligotrophic waters of the Southern California Bight and the eastern tropical North Pacific using microcosm manipulation experiments. Phytoplankton responses indicative of iron-light colimitation were found at several SCMs underlying macronutrient-limited surface waters in the eastern Pacific. Iron additions led to a shift in the size and taxonomic structure of the phytoplankton community, where large diatoms dominated what was formerly a diverse community of relatively small phytoplankton. The strongest and most ubiquitous responses of diatoms to iron addition were found under elevated light conditions, indicating that iron availability may have the greatest potential to affect SCM phytoplankton communities when light levels increase rapidly, such as during eddy events or with strong internal waves. The results show that iron influences phytoplankton community structure at SCMs, which would have consequences for nutrient cycling and carbon export within the lower euphotic zone.

Dupont, CL, Barbeau K, Palenik B.  2008.  Ni uptake and limitation in marine Synechococcus strains. Applied and Environmental Microbiology. 74:23-31.   10.1128/aem.01007-07   AbstractWebsite

Ni accumulation and utilization were studied in two strains of marine Synechococcus, isolated from both coastal (CC9311; clade I) and open-ocean (WTH8102; clade III) environments, for which complete genome sequences are available. Both strains have genes encoding an Ni-containing urease and when grown on urea without Ni become Ni-N colimited. The Ni requirements of these strains also depend upon the genomic complement of genes encoding superoxide dismutase (SOD). WH8102, with a gene encoding only an Ni-SOD, has a novel obligate requirement for Ni, regardless of the N source. Reduced SOD activity in Ni-depleted cultures of VM8102 supports the link of this strain's Ni requirement to Ni-SOD. The genome of CC9311 contains a gene for a Cu/Zn-SOD in addition to a predicted pair of Ni-SODs, yet this strain cannot grow without Ni on NO3- and can grow only slowly on NH4+ without Ni, implying that the Cu/Zn-SOD cannot completely replace Ni-SOD in marine cyanobacteria. CC9311 does have a greater tolerance for Ni starvation. Both strains increase their Ni uptake capabilities and actively bioconcentrate Ni in response to decreasing extracellular and intracellular Ni. The changes in Ni uptake rates were more pronounced in WH8102 than in CC9311 and for growth on urea or nitrate than for growth on ammonia. These results, combined with an analysis of fully sequenced marine cyanobacterial genomes, suggest that the growth of many marine Synechococcus and all Prochlorococcus strains is dependent upon Ni.

2007
Kwasnik, M, Fuhrer K, Gonin M, Barbeau K, Fernandez FM.  2007.  Performance, resolving power, and radial ion distributions of a prototype nanoelectrospray ionization resistive glass atmospheric pressure ion mobility spectrometer. Analytical Chemistry. 79:7782-7791.   10.1021/ac071226o   AbstractWebsite

In this article, we describe and characterize a novel ion mobility spectrometer constructed with monolithic resistive glass desolvation and drift regions. This instrument is equipped with switchable corona discharge and nanoelectrospray ionization sources and a Faraday plate detector. Following description of the instrument, pulsing electronics, and data acquisition system, we examine the effects of drift gas flow rate and temperature, and of the aperture grid to anode distance on the observed resolving power and sensitivity. Once optimum experimental parameters are identified, different ion gate pulse lengths, and their effect on the temporal spread of the ion packet were investigated. Resolving power ranged from an average value of 50 ms/ms for a 400-mu s ion gate pulse, up to an average value of 68 ms/ms for a 100-mu s ion gate pulse, and a 26-cm drift tube operated at 383 V cm(-1). Following these experiments, the radial distribution of ions in the drift region of the spectrometer was studied by using anodes of varying sizes, showing that the highest ionic density was located at the center of the drift tube. Finally, we demonstrate the applicability of this instrument to the study of small molecules of environmental relevance by analyzing a commercially available siderophore, deferoxamine mesylate, in both the free ligand and Fe-bound forms. Ion mobility experiments showed a dramatic shift to shorter drift times caused by conformational changes upon metal binding, in agreement with previous reversed-phase liquid chromatography observations.

Hopkinson, BM, Mitchell G, Reynolds RA, Wang H, Selph KE, Measures CI, Hewes CD, Holm-Hansen O, Barbeau KA.  2007.  Iron limitation across chlorophyll gradients in the southern Drake Passage: Phytoplankton responses to iron addition and photosynthetic indicators of iron stress. Limnology and Oceanography. 52:2540-2554.   10.4319/lo.2007.52.6.2540   AbstractWebsite

Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron-enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1-2 nmol L-1) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream high-chlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (F-v: F-m, sigma(PSII), and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.

Hopkinson, BM, Barbeau KA.  2007.  Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Marine Chemistry. 106:2-17.   10.1016/j.marchem.2006.02.008   AbstractWebsite

The organic and redox speciation of iron was examined in the strongly layered upper water column of the eastern tropical North Pacific, including oxic and sub oxic waters, in a region 100- 1300 km offshore. Suboxic conditions ([O-2] < 5 mu M) were found to affect the organic speciation of iron, and reduced dissolved iron, Fc(II), was present in the suboxic zone, but conditions were not sufficiently reducing to convert all iron to Fe(II). Dissolved iron concentrations in the suboxic zone were similar to concentrations found in oxic regions. Using a competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) method, natural ligands were found to have distinct characteristics in the oxic and suboxic waters with stronger ligands found in the suboxic zone. It is unusual to find stronger ligands below the euphotic zone, but their strength, logK(Fe'L) = 12.1-12.8, is within the range determined for surface ligands in other regions. These strong ligands may be the result of the unique chemistry of the suboxic zone stabilizing reduced or labile compounds, or they may be actively produced by microbes to enhance iron uptake. No onshore-offshore trends in ligand strength or concentration were detected suggesting the ligands may result from the inherent chemistry of the suboxic zone or production from denitrifiers, rather than the resident suboxic zone population of Prochlorococcus which were more abundant nearshore. A luminol-chemiluminescence based flow injection analysis (FIA) technique capable of detecting picomolar concentrations of Fe(II) was used to assess the redox state of iron in the suboxic zone and overlying oxic waters at a station 1300 km offshore. An elevated signal equivalent to 0.12-0.15 nM Fe(II), 21-24% of dissolved iron, was found only in the suboxic waters. Oxidation kinetics suggest that this Fe(II) is most likely produced by an in-situ process, as opposed to being transported from shelf sediment. The luminol-chemiluminescence Fe(II) method was systematically tested for inferences from reduced species potentially present in the suboxic zone to validate our Fe(II) results. Several species, V(IV) and V(111), produced significant signals, but considerations of the reducing state of the suboxic zone make it unlikely that reduced V is present. With additional information on the identity of the suboxic zone species provided by analysis of signal decay rate, it was determined that Fe(II) was the most reasonable source of the signal, and at minimum the chemiluminescence data allows us to set limits on the Fe(II) concentration in the offshore suboxic water column. (C) 2006 Elsevier B.V. All rights reserved.

King, AL, Barbeau K.  2007.  Evidence for phytoplankton iron limitation in the southern California Current System. Marine Ecology-Progress Series. 342:91-103.   10.3354/meps342091   AbstractWebsite

Observations of phytoplankton iron limitation in the world's oceans have primarily been confined to high-nutrient, low-chlorophyll (HNLC) regimes, found in the western equatorial and subarctic Pacific, Southern Ocean, and coastal upwelling zones off California and Peru. We investigated the potential for phytoplankton iron limitation in coastal transition zones (50 to 200 km offshore) of the southern California Current System, a weak upwelling regime that is relatively low in nutrients (< 4 mu mol nitrate 1(-1)) and low in chlorophyll (< 1 mu g chl a 1(-1)). In grow-out incubation experiments conducted during summer, July 2003 and 2004, phytoplankton responded to nanomolar iron additions, despite the non-HNLC initial conditions, Observed changes in phytoplankton and nutrient parameters upon iron addition were significant, although markedly lower in amplitude relative to typical grow-out experiments in HNLC regimes. While we cannot disprove alternate explanations for the observed limitation of phytoplankton growth, such as a proximate grazing control, our results indicate that phytoplankton growth in the southern California Current System is, at times, limited by the supply of iron. Based on our findings and the results of previous studies in this region, we suggest that phytoplankton biomass is generally limited by the supply of nitrate, while iron, directly or indirectly, influences macronutrient utilization, community species composition, and phytoplankton spatial and temporal distribution.

2006
Barbeau, K.  2006.  Photochemistry of organic iron(III) complexing ligands in oceanic systems. Photochemistry and Photobiology. 82:1505-1516.   10.1562/2006-06-16-ir-935   AbstractWebsite

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (> 99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O-2(-))The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimental/observational studies of the natural seawater ligand pool.

2003
Barbeau, K, Rue EL, Trick CG, Bruland KT, Butler A.  2003.  Photochemical reactivity of siderophores produced by marine heterotrophic bacteria and cyanobacteria based on characteristic Fe(III) binding groups. Limnology and Oceanography. 48:1069-1078. AbstractWebsite

Siderophores, high-affinity Fe(III) ligands produced by microorganisms to facilitate iron acquisition, might contribute significantly to dissolved Fe(III) complexation in ocean surface waters. In previous work, we demonstrated the photoreactivity of the ferric ion complexes of several alpha-hydroxy carboxylic acid-containing siderophores produced by heterotrophic marine bacteria. Here, we expand on our earlier studies and detail the photoreactivity of additional siderophores produced by both heterotrophic marine bacteria and marine cyanobacteria, making comparisons to synthetic and terrestrial siderophores that lack the alpha-hydroxy carboxylate group. Our results suggest that, in addition to secondary photochemical reaction pathways involving reactive oxygen species, direct photolysis of Fe(III)-siderophore complexes might be a significant source of Fe(II) and reactive Fe(III) in ocean surface waters. Our findings further indicate that the photoreactivity of siderophores is primarily determined by the chemical structure of the Fe(III) binding groups that they possess-hydroxamate, catecholate, or alpha-hydroxy carboxylate moieties. Hydroxamate groups are photochemically resistant regardless of Fe(III) complexation. Catecholates, in contrast, are susceptible to photooxidation in the uncomplexed form but stabilized against photooxidation when ferrated. alpha-Hydroxy carboxylate groups are stable as the uncomplexed acid, but when coordinated to Fe(III), these moieties undergo light-induced ligand oxidation and reduction of Fe(III) to Fe(II). These photochemical properties appear to determine the reactivity and fate of Fe(III)-binding siderophores in ocean surface waters, which in turn might significantly influence the biogeochemical cycling of iron.

2002
Barbeau, K, Zhang GP, Live DH, Butler A.  2002.  Petrobactin, a photoreactive siderophore produced by the oil-degrading marine bacterium Marinobacter hydrocarbonoclasticus. Journal of the American Chemical Society. 124:378-379.   10.1021/ja0119088   AbstractWebsite

Petrobactin is a bis-catecholate, α-hydroxy acid siderophore produced by the oil-degrading marine bacterium Marinobacter hydrocarbonoclasticus. The Fe(III)-complexed form of petrobactin is photoreactive in natural sunlight, mediated by the Fe(III)-citrate moiety. The reaction results in decarboxylation of the petrobactin ligand and reduction of Fe(III) to Fe(II). This report is one of the first to show the photoreactivity of Fe(III)-siderophores mediated by the ferric ion-α-hydroxy acid group. The demonstration of light-mediated decarboxylation of an Fe(III)-siderophore complex raises questions about a possible functional role for photoreactivity in siderophore-mediated iron uptake.

2001
Barbeau, K, Rue EL, Bruland KW, Butler A.  2001.  Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands. Nature. 413:409-413.   10.1038/35096545   AbstractWebsite

Iron is a limiting nutrient for primary production in large areas of the oceans(1-4). Dissolved iron(III) in the upper oceans occurs almost entirely in the form of complexes with strong organic ligands(5-7) presumed to be of biological origin(8,9). Although the importance of organic ligands to aquatic iron cycling is becoming clear, the mechanism by which they are involved in this process remains uncertain. Here we report observations of photochemical reactions involving Fe(III) bound to siderophores-high-affinity iron(III) ligands produced by bacteria to facilitate iron acquisition(10-12). We show that photolysis of Fe(III)-siderophore complexes leads to the formation of lower-affinity Fe(III) ligands and the reduction of Fe(III), increasing the availability of siderophore-bound iron for uptake by planktonic assemblages. These photochemical reactions are mediated by the alpha -hydroxy acid moiety, a group which has generally been found to be present in the marine siderophores that have been characterized(13-15). We suggest that Fe(III)-binding ligands can enhance the photolytic production of reactive iron species in the euphotic zone and so influence iron availability in aquatic systems.

Barbeau, K, Kujawinski EB, Moffett JW.  2001.  Remineralization and recycling of iron, thorium and organic carbon by heterotrophic marine protists in culture. Aquatic Microbial Ecology. 24:69-81.   10.3354/ame024069   AbstractWebsite

To characterize trace metal cycling in marine systems as mediated by heterotrophic protists, we conducted a series of laboratory experiments in 2-organism model systems consisting of bacteria and protistan grazers. Trace metal isotopes (Fe-59 and Th-234),C-14, and bulk organic carbon measurements were used to follow the chemical transformation of bacterial carbon and associated trace metals by several different grazer species. Results indicate that grazers were able to cause repartitioning of Th and regeneration of Fe from bacterial prey into the dissolved phase (<0.2 m), even in particle-rich laboratory cultures. For both Th and Fe, protist grazing led to the formation of relatively stable dissolved and colloidal metal-organic species. Metal/carbon ratios of the particle pool in some model systems with grazers were significantly altered, indicating a decoupling of trace metal and organic carbon cycling through the grazing process. Different protist species exhibited substantial variation (up to a factor of 10) in their ability to quantitatively remobilize trace metals from bacterial prey. The implications of these findings for trace metal cycling in marine systems are discussed.

2000
Barbeau, K, Moffett JW.  2000.  Laboratory and field studies of colloidal iron oxide dissolution as mediated by phagotrophy and photolysis. Limnology and Oceanography. 45:827-835. AbstractWebsite

In a previous work, we have employed colloidal ferrihydrite impregnated with an inert radiotracer to probe the mechanistics of iron redox cycling in seawater via phagotrophic and photochemical processes. This paper reports further studies using the inert tracer technique, directed towards obtaining a more quantitative sense of the importance of phagotrophy relative to photolysis as a pathway for the production of bioavailable iron in oxygenated seawater. Our results indicate a maximal (i.e., near-surface at noon) rate of 12% per day for the photochemically-mediated dissolution of colloidal ferrihydrite. Protozoan-mediated dissolution of the same iron oxide phase proceeds at a rate ranging from 1-6% per day, depending on grazing turnover rates. Thus, while photolysis should dominate the redox cycling of refractory iron solids in near-surface waters under bright daytime conditions, phagotrophy is likely to be a more important process overall when the entire euphotic zone is considered on a time-averaged basis.

1998
Barbeau, KA, Moffett JW.  1998.  Dissolution of Iron Oxides by Phagotrophic Protists:  Using a Novel Method To Quantify Reaction Rates. Environmental Science & Technology. 32:2969-2975.: American Chemical Society   10.1021/es9802549   AbstractWebsite

In previous work, we have reported the dissolution of iron oxides within the acidic food vacuoles of marine protozoan grazers as evidence of a novel mechanism for the conversion of refractory iron solids to more labile forms in oxic surface waters. This paper expands upon those initial studies and presents a new technique to study the reaction of iron oxides in seawater, based on the synthesis of colloidal ferrihydrite containing an inert tracer. Measuring the accumulation of the tracer in the dissolved phase enables the determination of the rate and extent of iron oxide reaction, even for kinetically slow processes and regardless of the fate of iron in the system. The validity of the method as a means of following the reaction of iron oxides in seawater is shown here in a series of co-dissolution studies and in several photochemical kinetics experiments. In laboratory studies of the dissolution of colloidal ferrihydrite by protozoan grazers, the inert tracer method enables an improved estimate of the rate of protozoan-mediated iron oxide dissolution, confirming our previous results and providing a useful tool for further studies of phagotrophy as a reaction pathway for refractory iron.